A number of cardiac rhythm management products are available for the use in diagnosis and treatment of various conditions. These may include, for example, subcutaneous, transvenous, or intracardiac therapy devices such as pacemakers, defibrillators and resynchronization devices. Implantable, external and/or wearable cardiac monitors are also available. External or wearable therapy products may include defibrillator vests and external pacemakers, as well as automatic external defibrillators.
FIG. 1, which is taken from Ellenbogen, et al. in CLINICAL CARDAIC PACING AND DEFIBRILLATION, 2nd Ed. (W.B. Saunders Co. 2000), at 201, shows the frequency of raw cardiac signals and non-cardiac myopotentials. The signals include T-waves, which represent ventricular repolarization and have a frequency content in the range of about 3-9 Hz or so. R-waves are also indicated and represent ventricular depolarization; the R-wave frequency range is typically from about 20 Hz to about 40 Hz. P-waves, representing atrial depolarization, are still higher frequency, in the range of about 30-70 Hz. Myopotentials, representing non-cardiac muscle activity, tend to have frequency content of 90 Hz and above.
T-wave filtering may be desirable for systems subject to a risk of T-wave overdetection, which can lead to overcounting of cardiac cycles and possibly to inappropriate therapy. While filtering the T-wave out may reduce potential overdetection and inappropriate therapy, it is also necessary to ensure that filtering directed at the T-waves does not lead to undersensing of tachyarrhythmias such as ventricular fibrillation or polymorphic ventricular tachycardia. Such arrhythmias are often detected by monitoring for and counting the R-waves.
The concern arises because the signals for T and R waves are so close together in the frequency domain. For example, a first order high pass filter with a corner frequency at 10 Hz, which would attenuate the 3-9 Hz T-wave, will also attenuate a signal at 15 Hz by ten to twenty percent, or more, which can be significant. Given that barely a decade (a tenfold increase in frequency) separates the noted signals, the effect of a filter directed at one set of signals on other signals should be monitored.
New and alternative approaches to filtering control are desired.